The present invention relates to a method of detecting symbols of an OFDM (Orthogonal Frequency Division Multiplexing) signal transmitted from another communication apparatus, a communication apparatus including an OFDM symbol detecting unit in a power line communication, etc.
A transmission method of using a plurality of sub-carriers, such as an OFDM method, has a big advantage in realizing high quality communication even though a transmission channel is in a severe environment. Therefore, this transmission method is also used for wired communication such as power line communication as well as wireless communication.
In a multi-carrier communication apparatus for carrying out communication using a plurality of sub-carriers, a transmitting side converts bit data to be transmitted into symbol data, executes symbol mapping on the symbol data, converts the symbol data into data on a time domain by executing an inverse FFT conversion or a inverse wavelet conversion, executes a parallel-serial conversion, converts the data into baseband analog signals by executing a DA conversion, and transmits the converted baseband analog signals. In addition, a receiving side converts the received signals into digital signal by executing an AD conversion, executes a serial-parallel conversion, converts the data into data on a frequency domain by executing an FFT conversion or a wavelet conversion, and executes demapping so as to obtain reception bit data.
The multi-carrier communication apparatus has a carrier detecting function of determining whether another communication apparatus transmits signals to a transmission line. When another communication apparatus does not use the transmission line, the multi-carrier communication apparatus executes a transmission process in response to a transmission request.
As a carrier detecting method in the multi-carrier communication apparatus, there is known a method of detecting whether a multi-carrier signal exists by calculating an average value of all sub-carriers on the basis of signals obtained by converting digital waveform data on the time-domain into data on the frequency-domain, for example (see patent literature 1). In the carrier detection, a high detection accuracy can be obtained by using a correlation between adjacent sub-carriers in a frequency domain on the basis of an OFDM property that the plurality of sub-carriers is arranged at the same interval.
As another example of the carrier detecting method, there is known a method of detecting whether a multi-carrier signal exists by determining the correlation on the basis of a distribution of constellation points on the complex coordinates or by calculating the correlation between the alternately adjacent sub-carriers when calculating the correlation between the adjacent sub-carriers (see patent literature 2). In the OFDM, phase differences between all the adjacent sub-carriers are the same in a preamble of a frame header. Therefore, by evaluating the distribution of the constellation points on the complex coordinates, it can be determined whether the multi-carrier signal exists with high accuracy.
[Patent literature 1] JP-A-2005-57644
[Patent literature 2] JP-A-2007-281577
In the method of executing the carrier detection using the average value of all the sub-carriers, as in patent literature 1, a problem arises in that the carrier detection is possible in a state where a narrowband noise occurs in a part of frequency used in communication but the carrier detection cannot be executed since it is difficult to deal with a state where large level noises overlap with each other across a broadband spectrum.
Also, in the method of executing the carrier detection by calculating the inter-carrier correlation by the use of the distribution of the constellation points on the complex coordinates, as in patent literature 2, the carrier detection can be executed with high accuracy by the use of the inter-carrier correlation. However, the signal cannot be received since the half or more of the sub-carriers are submerged in noise, for example. Therefore, a problem arises in that the carrier detection cannot be normally executed in many cases since there is a low probability that the phase difference between the adjacent sub-carriers falls within a predetermined range in a state where only a few of the sub-carriers are received, for example.
In the known carrier detecting methods, the carrier detection cannot be normally executed even with the existence of the signals in some cases, when the large level noises occur across in the broadband spectrum. In particular, under an environment where transmission feature vary due to the connection of another communication apparatus to the power line serving as a transmission line or a noise occurs due to another communication apparatus, as in the power line communication, it cannot be normally determined whether a reception target signal exists or it takes significant time to determine whether the received signal is a reception target signal.